Serious problem with QGLWidget

[al101]

Hello

I'm looking to write an opengl based editor using QT but I'm experiencing a problem with QGLWidget. I have implemented a camera class and it moves smoothly when I move it 'programmatically' and from input using the mouseMoveEvent, but when I update the camera using input from keyPressEvent it is erratic and shows what looks like a double image of the previous frame. I've subclassed the QGLWidget and placed it on my main window and tried adding the keyPressEvent function to the opengl widget class and as a function of main window but the same problem remains.

Has anybody else experienced this problem, which I belive is caused by the high number of keyboard events being sent, and know of a way to work around it for smooth keyboard input?

I am currently using version 4.3.0beta/Mingw with QDevelop and have an example program that allows you to switch from one input method to another as a test. It contains all the source code and project file too. The download is available here at http://www.alsprogrammingresource.com/Test.zip

Regards
Alan

[wysota]

I suggest you start by adding a timer to check for key presses instead of handling them directly in keyPressEvent to get a steady movement. In keyPress/keyRelease events only flag that a key has been pressed or released and then handle the actual change in the timer timeout. Maybe this will be enough to make it work correctly.

[al101]

Thanks wysota

I'll certainly give that a try. Polling the key press events periodically may be the only work around but I was wondering if it was a common problem with a standard solution. I'll see what happens and post the results if successful.

[wysota]

"Standard solution" is to use a timer. You're currently using a non-standard one

[al101]

ok, I've added an array of flags to record which keys were pressed:

bool keypress[256];

and changed the keyPressEvent and keyReleaseEvent to:

void MainWindowImpl::keyReleaseEvent( QKeyEvent *e )
 
{   
 
    switch( e->key() )
 
    {
 
    	if (keyboardControl)
 
    	{    		
 
			case Qt::Key_W:
 
			case Qt::Key_S:
 
			case Qt::Key_A:
 
			case Qt::Key_D:
 
			case Qt::Key_Q:
 
			case Qt::Key_E:
 
				keypress[e->key()] = false; 
 
	        break;
 
        }
 
    }
 
}
 
 
 
void MainWindowImpl::keyPressEvent( QKeyEvent *e )
 
{   
 
    switch( e->key() )
 
    {
 
    	if (keyboardControl)
 
    	{    		
 
		case Qt::Key_W:
 
		case Qt::Key_S:
 
		case Qt::Key_A:
 
		case Qt::Key_D:
 
		case Qt::Key_Q:
 
		case Qt::Key_E:
 
			keypress[e->key()] = true; 
 
	        break;
 
	}
    }
}

void MainWindowImpl::keyReleaseEvent( QKeyEvent *e )

{   

    switch( e->key() )

    {

    	if (keyboardControl)

    	{    		

			case Qt::Key_W:

			case Qt::Key_S:

			case Qt::Key_A:

			case Qt::Key_D:

			case Qt::Key_Q:

			case Qt::Key_E:

				keypress[e->key()] = false; 

	        break;

        }

    }

}

   

void MainWindowImpl::keyPressEvent( QKeyEvent *e )

{   

    switch( e->key() )

    {

    	if (keyboardControl)

    	{    		

		case Qt::Key_W:

		case Qt::Key_S:

		case Qt::Key_A:

		case Qt::Key_D:

		case Qt::Key_Q:

		case Qt::Key_E:

			keypress[e->key()] = true; 

	        break;

	}
    }
}

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and changed the time out routine to:

void OGLWidget::timeOutSlot()
 
{
 
	if (keypress[Qt::Key_W])
 
		camera[currentCamera].Movement_z -= 0.01;
 
 
 
	if (keypress[Qt::Key_S])
 
		camera[currentCamera].Movement_z += 0.01;
 
 
 
	if (keypress[Qt::Key_A])
 
		camera[currentCamera].Movement_x -= 0.01;
 
 
 
	if (keypress[Qt::Key_D])
 
		camera[currentCamera].Movement_x += 0.01;
 
 
 
	if (keypress[Qt::Key_Q])
 
		camera[currentCamera].Movement_y -= 0.01;
 
 
 
	if (keypress[Qt::Key_E])
 
		camera[currentCamera].Movement_y += 0.01;
 
 
 
    updateGL();
 
}

void OGLWidget::timeOutSlot()

{

	if (keypress[Qt::Key_W])

		camera[currentCamera].Movement_z -= 0.01;



	if (keypress[Qt::Key_S])

		camera[currentCamera].Movement_z += 0.01;



	if (keypress[Qt::Key_A])

		camera[currentCamera].Movement_x -= 0.01;



	if (keypress[Qt::Key_D])

		camera[currentCamera].Movement_x += 0.01;



	if (keypress[Qt::Key_Q])

		camera[currentCamera].Movement_y -= 0.01;



	if (keypress[Qt::Key_E])

		camera[currentCamera].Movement_y += 0.01;

		

    updateGL();

}

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but the results are still erratic, how would I go about polling key press events using a timer or can you suggest an example or tutorial that I could refer to?

[jacek]

switch( e->key() )
{
	if (keyboardControl)
	{
		case Qt::Key_W:
		...
	}
}

switch( e->key() )
{
	if (keyboardControl)
	{
		case Qt::Key_W:
		...
	}
}

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In this case "if(keyboardControl)" has no effect, as switch simply jumps to appropriate case clause.

[al101]

thanks, I noticed this after I had released the test program. It didn't play a part in the problem so I just left it.

[wysota]

Instead of the array you may want to try QSet<Qt::Key> and possibly also QSet<Qt::KeyboardModifier>. It should give you more flexibility. If not, then I'd like to notice that you're wasting 7 out of 8 bits of each cell in the array which gives a total waste of 224 bytes

[al101]

ic, good to know

On the keyboard input subject I might add that I have been downloading every QT program that displays an QGLWidget that I could find. A prime example of the problem can be seen clearly when viewing the examples of libQGL Viewer available here http://artis.imag.fr/~Gilles.Debunne/QGLViewer/

[wysota]

Could you be more specific?

[al101]

Sure, if you download any of the examples on that site and use the arrow keys to move the camera left and right you will see that the problem is very obvious. I simply can't find any application with a QGLWidget with smooth scrolling based on keyboard input.

[wysota]

Is this smooth enough for you (it rotates instead of scrolling, but the idea remains...)?

[al101]

Back from sleep, work...

Thanks for the example wysota. I just found that the changes in my third post worked to solve the problem, but I also had to remove the timing routine that I introduced as a fix.

So again, thanks for working with me on this. I'll post the link of a working example or tutorial soon.